1. Field of the Invention
The present invention relates generally to an active suspension system which detects a predetermined vehicular driving condition and performs suspension control for regulating vehicular height level and suppressing vehicular attitude variation, such as pitching, rolling and so forth.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit for controlling the valve position. The pressure control valve controls the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and thereby controls suspension characteristics.
On the other hand, European Patents 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-154413 discloses a vehicular height regulation technology in an active suspension system. The disclosed system allows independent height adjustment at left and right rear suspension systems by means of pressure control valves operable independent of each other. In contrast to this, the shown system employs a common pressure control valve for adjusting height level at left and right front suspension systems. In the practical operation of the shown active suspension system, the height adjustment for the rear suspension systems is performed in advance of adjustment of height level at the front suspension systems. By providing difference in timing of height adjustment between the front and rear suspension systems, hunting in fine adjustment of the vehicular height can be successfully prevented.
On the other hand, with the prior proposed construction, anti-pitching can be performed successfully for effectively suppressing vehicular pitching motion. However, an anti-rolling operation cannot be performed as readily because of impossibility of independent control of the height level at the front suspension systems. Particularly, when the vehicular passenger or luggage load is uneven in transverse direction to cause lateral shifting of the gravity center of the vehicle, the fluid pressure in the working chamber of active cylinders in rear suspension systems, which are controlled independently of the other, even while no lateral acceleration is exerted on the vehicle. By difference of the initial fluid pressure, range of pressure adjustment at the left and rear suspension systems are differentiated to each other. This causes difference of roll stabilizing capacity for left-hand and right-hand turns for degrading drivability.